This invention relates to slide fasteners and particularly to stringer tapes of a warp-knitted structure for mounting thereon rows of interlocking fastener elements.
There are known various types of warp-knitted tapes for use in the field of slide fasteners or zippers. Warp-knitted tapes of the known type onto which interlocking fastener elements are sewn are basically constructed with longitudinally extending chain stitches which form a multiplicity of wales and transversely extending lapping threads laid in to connect the wales coursewise so as to solve the inherent problem of "stretch" in the knitted fabric which occurs when subjected to external stresses exerted in normal use of the slide fastener. With these prior art warp-knitted tapes, the arrangement of the laid-in lapping threads at the element-supporting tape edge is that the lapping threads form u-turns in every two courses at the outermost wale adjacent to a line of sewing stitches which secures the fastener elements to the tape edge. In other words, the element-supporting tape edge includes a first group of courses where lapping threads are located and a second group of courses free of lapping threads, the second courses tending to yield themselves at the tape edge portion to a greater stretch weftwise than the first courses when lateral stresses are exerted on the tape fabric.
This has led to the drawback that the sewing stitches run longitudinally across and pass through these two different groups of courses at the element-supporting edge, thereby creating irregularities in the positional stability of a row of fastener elements secured to the tape edge during the use of the slide fastener. The prior art tape structures are therefore prone to reduce the mechanical strength of the resulting slide fastener against stresses tending to bend or split apart the coupled slide fastener.
One solution to this drawback would be to see to it that the sewing needle passes exactly through only the courses at the tape edge where the inlaid lapping threads are devoid and that the sewing pitch registers with such courses. However, it would be difficult to reduce this proposal to practice when taking into account the inherent flexibility and stretch of the knitted tape and the shape of a given fastener element.